I'm breaking radio silence because of a brilliantly lucid article (from the subscription-only Barron's site) found for us by Michael Panzner.
Vitaliy Katsenelson explains that the current average price-earnings ratio may seem cheap, but that's because recent profit margins have been well above the 8.5% trend. Even allowing for a shift since 1980 away from industry towards the higher-margin service sector, the present 11.9% profit margin should be seen against a longer-term background figure of around 8.9 - 9.2%, which if current p/e ratios continue would imply a downward stock price correction of 22 -25%.
This chimes with Robert McHugh's "Dow 9,000" prediction from last July. And in many fields it's usual for overshoot to occur in the process of regression to a mean, so if it holds true in this case we could see even deeper temporary lows.
Day traders, be warned: this piste is a Black Run.